A passive optical network (PON) is a point-to-multipoint network architecture in which unpowered optical splitters are used to enable a single optical fibre to serve multiple premises. A PON typically includes an Optical Line Terminal (OLT) at the service provider's central office connected to a number (typically 32-128) of Optical Network Terminals (ONTs), each of which provides an interface to customer equipment.
In operation, downstream signals are broadcast from the OLT to the ONTs on a shared fibre network. Various techniques, such as encryption, can be used to ensure that each ONT can only receive signals that are addressed to it. Upstream signals are transmitted from each ONT to the OLT, using a multiple access protocol, such as time division multiple access (TDMA), to prevent “collisions”.
A Wavelength Division Multiplexing PON, or WDM-PON, is a type of passive optical network in which multiple optical wavelengths are used to increase the upstream and/or downstream bandwidth available to end users. FIG. 1 is a block diagram illustrating a typical WDM-PON system. As may be seen in FIG. 1, the OLT 4 comprises a plurality of transceivers 6, each of which includes a light source 8 and a detector 10 for sending and receiving optical signals on respective wavelength channels, and an optical combiner/splitter 12 for combining light from/to the light source 8 and detector 10 onto a single optical fibre 14. The light source 8 may be a conventional laser diode such as, for example, a distributed feed-back (DFB) laser, for transmitting data on the desired wavelength using either direct laser modulation, or an external modulator (not shown) as desired. The detector 10 may, for example, be a PIN diode for detecting optical signal received through the network. An optical mux/demux 16 (such as, for example, a Thin-Film Filter—TFF) is used to couple light between each transceiver 6 and an optical fibre trunk 18, which may include one or more passive optical power splitters (not shown).
A passive remote node 20 serving one or more customer sites includes an optical mux/demux 22 for demultiplexing wavelength channels (λ1 . . . λn) from the optical trunk fibre 18. Each wavelength channel is then routed to an appropriate branch port 24 which supports a respective WDM-PON branch 26 comprising one or more Optical Network Terminals (ONTs) 28 at respective customer premises. In the WDM-PON of FIG. 1, one of the branches 26a includes a pair of ONTs 28, which share common uplink and downlink wavelength channels using, for example, a time division multiple access scheme well known in the art. Typically, each ONT 28 includes a light source 30, detector 32 and combiner/splitter 34, all of which are typically configured and operate in a manner mirroring that of the corresponding transceiver 6 in the OLT 4.
Typically, the wavelength channels (λ1 . . . λn) of the WDM-PON are divided into respective channel groups, or bands, each of which is designated for signalling in a given direction. For example, C-band (e.g. 1530-1565 nm) channels may be allocated to uplink signals transmitted from each ONT 28 to the OLT 4, while L-band (e.g. 1565-1625 nm) channels may be allocated to downlink signals from the OLT 4 to the ONT(s) 26 on each branch 26. In such cases, the respective optical combiner/splitters 12,34 in the OLT transceivers 6 and ONTs 286 are commonly provided as passive optical filters well known in the art.
The WDM-PON illustrated in FIG. 1 is known, for example, from “Low Cost WDM PON With Colorless Bidirectional Transceivers”, Shin, DJ et al, Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. With this arrangement, each branch 26 is allocated a predetermined pair of wavelength channels, comprising an L-band channel for downlink signals transmitted from the OLT 4 to the branch 26, and a C-band channel for uplink signals transmitted from the ONT(s) 28 of the branch 26 to the OLT 4. The MUX/DEMUX 16 in the OLT 4 couples the selected channels of each branch 26 to a respective one of the transceivers 6. Consequently, each transceiver 6 of the ONT is associated with one of the branches 26, and controls uplink and downlink signalling between the ONT 4 and the ONT(s) 28 of that branch 26. Each transceiver 6 and ONT 28 is rendered “colorless”, by using reflective light sources 8, 30, such as reflective semi-conductor optical amplifiers; injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. With this arrangement, each light source 8, 30 requires a “seed” light which is used to produce the respective downlink/uplink optical signals. In the system of FIG. 1, the seed light for downlink signals is provided by an L-band broadband light source (BLS) 34 via an L-band optical circulator 36. Similarly, the seed light for uplink signals is provided by a C-band broadband light source (BLS) 38 via a C-band optical circulator 40. As is known in the art, the BLSs 34 and 38 may be broadband light emitting sources that generate a continuous spectrum, such as a Light Emitting Diode (LED), or may be a multi-frequency laser source which generates a plurality of narrow-band lights.
WDM-PONs suffer limitations in that the fibre trunk 18 and the BLSs 34,38 constitute single points of failure of the entire network. A failure of any one of these components effectively disconnects all subscribers.
A typical method for implementing WDM-PON protection is to duplicate the WDM-PON system of FIG. 1, with both systems being connected to the same ONTs 26. A less expensive alternative, which provides light source protection, is illustrated in FIG. 2. In the system of FIG. 2, the L-band and C-band BLSs 34 and 38 are protected by duplicating each BLS, and then routing light from a selected one of the two BLSs (for both uplink and downlink seed light) using an optical switch 46, as shown in FIG. 2. While this arrangement does provide protection for the L-band and C-band light sources, it also significantly increases the cost and complexity of the OLT 4.